1. Field of the Invention
The present invention relates generally to an exhaust emission purifying apparatus for reductively purifying nitrogen oxides (NOx) in the exhaust gas using the liquid reducing agent, and in particular, to a technology especially for achieving the effective filtration of a liquid reducing agent while suppressing the freezing thereof in a reducing agent container. The present invention further relates to the reducing agent container having an improved structure thereof capable of realizing the effective filtration of a liquid reducing agent while suppressing the freezing thereof.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2000-27627 discloses an exhaust emission purifying apparatus as one proposal for a catalytic converter purifying system for eliminating NOx contained in the exhaust gas of an engine. In such an exhaust emission purifying apparatus, a liquid reducing agent according to engine operating conditions is injection-supplied to the flow of the exhaust emission in an upstream portion of a reduction catalytic converter disposed in an engine exhaust emission system via a reducing agent supply device which is controlled by a control unit, so that NOx in the exhaust gas and the liquid reducing agent are subjected to the catalytic-reduction reaction, to thereby purify NOx into harmless components. Here, for the reduction reaction, ammonia having the good reactivity to NOx is used, and as the liquid reducing agent, the urea aqueous solution, which is hydrolyzed with the exhaust heat and the water vapor in the exhaust gas to generate ammonia, is used.
However, when foreign substances are mixed into the liquid reducing agent, a strainer incorporated in the reducing agent supply device is clogged in a short time, resulting in that the liquid reducing agent cannot be successfully injection-supplied to the exhaust upstream of the reduction catalytic converter. As foreign substance mixing routes, there can be assumed various situations, such as, spatters during the manufacturing process of the reducing agent container, mixing of fibrous material having detached from an operator's glove or various insects when the liquid reducing agent is replenished, and the like.
There is known a technology for disposing a strainer in engine fuel piping. However, if this known technology is applied to the exhaust emission purifying apparatus without any modification, there is caused the following problem. Namely, in a cold region, such as Hokkaido island of Japan, if a moving vehicle mounted with the exhaust emission purifying apparatus travels, since the whole of strainer is exposed to the wind during traveling, there is a possibility that the liquid reducing agent in the strainer might be frozen. If the liquid reducing agent in the strainer is frozen, the liquid reducing agent cannot be successfully injection-supplied to the exhaust upstream of the reduction catalytic converter, resulting in that a function of the exhaust emission purifying apparatus cannot be achieved. Further, a consideration could be made on a configuration in which the strainer is attached to a replenishment port for the reducing agent container. However, since there is a possibility that the strainer is broken at the time of replenishment, such a configuration is not adoptable in easy.